Semiconductor device and method of driving transistors

ABSTRACT

When a driving unit ( 100 ) charges gate input capacitance ( 6 ) of an IGBT ( 7 ), the gate input capacitance ( 6 ) accumulates electric charges which are accumulated therein when the driving unit ( 100 ) discharges the gate input capacitance ( 6 ). Therefore, it is possible to reduce the amount of electric charges to be supplied to the gate input capacitance ( 6 ) by the driving unit ( 100 ) until the charge of the gate input capacitance ( 6 ) is completed. As a result, it is possible to reduce the required power capacity of a control power supply ( 15   a ). Further, since the electric charges accumulated in the gate input capacitance ( 6 ) are effectively used, it is possible to ensure power savings of a semiconductor device. Thus provided are a semiconductor device and a method of driving a transistor, in which electric charges accumulated in gate input capacitance of an insulated gate transistor are effectively utilized, to reduce required power capacity of a power supply for driving the transistor and ensure power savings of the semiconductor device on the whole.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, and more particularly to a semiconductor device which charges and discharges gate input capacitance of an insulated gate transistor and a method of driving the transistor.

2. Description of the Background Art

FIG. 14 is a view showing a configuration of a semiconductor device in a first background art. As shown in FIG. 14, the semiconductor device of the first background art comprises an IGBT (Insulated Gate Bipolar Transistor) 7 which is an insulated gate transistor, a protection diode 8 working against a reverse voltage applied between an emitter and a collector of the IGBT 7, a gate driving circuit 3, a control power supply 15 a which outputs a voltage of +15 V, a control power supply 15 b which outputs a voltage of −15 V, a transistor 4 a having a collector connected to the control power supply 15 a, a transistor 4 b having an emitter connected to the control power supply 15 b and a resistor 5. Gate input capacitance 6 is parasitic capacitance generated structurally between a gate and the emitter of the IGBT 7.

In the semiconductor device of FIG. 14, in order to turn the IGBT 7 on, first, the gate driving circuit 3 turns the transistor 4 a on and turns the transistor 4 b off. When the transistor 4 a is turned on, the control power supply 15 a supplies the gate of the IGBT 7 with the voltage of +15 V. At this time, since the gate input capacitance 6 is present between the gate and emitter of the IGBT 7, electric charges are supplied by the control power supply 15 a to the gate input capacitance 6, thereby producing a gate current. As the gate input capacitance 6 is being charged, a gate voltage of the IGBT 7 increases and the IGBT 7 is turned on when the gate voltage of the IGBT 7 reaches a threshold voltage or more. After that, the charge of the gate input capacitance 6 of the IGBT 7 is completed, the gate voltage becomes about +15 V and the gate current almost stops flowing therein. Further, in some cases, the gate current flowing during a period from the start to end of the charge of the gate input capacitance 6 is simply referred to as “charging current”.

When the IGBT 7 is turned off, the gate driving circuit 3 turns the transistor 4 a off and turns the transistor 4 b on. When the transistor 4 b is turned on, the control power supply 15 b supplies the gate of the IGBT 7 with the voltage of −15 V. At this time, since the electric charges are accumulated in the gate input capacitance 6 of the IGBT 7, the electric charges are extracted by the control power supply 15 b, thereby producing the gate current in a direction reverse to that of the case where the IGBT 7 is turned on. As the gate input capacitance 6 is being discharged, the gate voltage of the IGBT 7 decreases and the IGBT 7 is turned off when the gate voltage of the IGBT 7 becomes less than the threshold voltage. After that, the discharge of the gate input capacitance 6 of the IGBT 7 is completed, the gate voltage becomes about −15 V and the gate current almost stops flowing therein. Further, in some cases, the gate current flowing during a period from the start to end of the discharge of the gate input capacitance 6 is simply referred to as “discharging current”.

FIG. 15 is a view showing a relation of the gate voltage V_(ge), the gate current I_(ge) and a collector current I_(C) at the time when the IGBT 7 of FIG. 14 is turned on, and the gate current I_(ge) corresponds to the charging current of the IGBT 7. As shown in FIG. 15, when the transistor 4 a is turned on, the gate current I_(ge) for charging the gate input capacitance 6 largely flows, in other words, the charging current largely flows, and after that, as the gate input capacitance 6 is being charged, in other words, as the gate voltage V_(ge) increases, the gate current I_(ge) decreases and when the gate voltage V_(ge) becomes almost equivalent to the voltage outputted from the control power supply 15 a, the gate current I_(ge) almost stops flowing.

As discussed above, with the charging current supplied by the control power supply 15 a, the IGBT 7 is turned on. In other words, the control power supply 15 a needs power capacity to supply the charging current. Similarly, in order to turn the IGBT 7 off, the control power supply 15 b needs power capacity to supply the discharging current. Then, in order to increase a rated current between the emitter and collector of the IGBT 7, it is usually necessary to increase the chip size of the IGBT 7, which leads to an increase of the gate input capacitance 6. For this reason, driving the IGBT 7 having a large rated current needs the control power supplies 15 a and 15 b having large power capacity. Further, when the IGBT 7 is used for an inverter device, as the operating frequency of the inverter device, i.e., the switching frequency of the IGBT 7 becomes higher, the charging current flowing per unit time becomes larger. For this reason, faster driving the IGBT 7 needs the control power supplies 15 a and 15 b having large power capacity. Thus, as the rated current of the IGBT 7 increases, and as faster driving of the IGBT 7 is desired, the power capacity of the control power supplies 15 a and 15 b required to drive the IGBT 7 increases.

The increase in power capacity of the control power supplies 15 a and 15 b as discussed above leads to an increase in cost and packaging volume of the control power supplies 15 a and 15 b. For this reason, in recent years when it is desired to reduce the cost and size of semiconductor devices, reduction of required power capacity of the control power supplies 15 a and 15 b is needed.

Then, a second background art is proposed, where the required power capacity of the control power supplies 15 a and 15 b are reduced. FIG. 16 is a view showing a configuration of a semiconductor device in the second background art. The semiconductor device of the second background art further comprises capacitors 11 a and 11 b besides the configuration of the first background art discussed above.

As shown in FIG. 16, when both the transistors 4 a and 4 b are in an off state, the capacitors 11 a and 11 b are charged by the control power supplies 15 a and 15 b. Then, in order to turn on the IGBT 7, when the gate driving circuit 3 turns the transistor 4 a on and turns the transistor 4 b off, electric charges accumulated in the capacitor 11 a go through the transistor 4 a to be supplied to the gate input capacitance 6 of the IGBT 7 where the supplied electric charges are accumulated. By the way, the gate current I_(ge) of the IGBT 7 largely flows first when the transistor 4 a is turned on and after that gradually decreases, as shown in FIG. 15. In short, the control power supply 15 a of the first background art needs current supplying capability to produce the peak value of the gate current I_(ge) as shown in FIG. 15. In the above-discussed second background art, since the electric charges accumulated in the capacitor 11 a are supplied to the gate input capacitance 6 when the transistor 4 a is turned on, the current supplied directly to the gate input capacitance 6 by the control power supply 15 a decreases. For this reason, the control power supply 15 a of the second background art does not need the current supplying capability to produce the peak value of the gate current I_(ge) as shown in FIG. 15. In other words, it is possible to reduce the required power capacity of the control power supply 15 a.

Further, in order to turn off the IGBT 7, when the gate driving circuit 3 turns the transistor 4 a off and turns the transistor 4 b on, the electric charges accumulated in the gate input capacitance 6 go through the transistor 4 b to be supplied to the capacitor 11 b. For this reason, like for the control power supply 15 a, it is possible to reduce the required power capacity of the control power supply 15 b. Furthermore, almost the same technique as the second background art shown in FIG. 16 is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-7617.

In the second background art, however, since the capacitors 11 a and 11 b are charged by the control power supplies 15 a and 15 b, respectively, the total amount of electric charges supplied by the control power supplies 15 a and 15 b is not reduced when the IGBT 7 is turned on or off.

Then, a third background art is proposed, where energy accumulated in the gate input capacitance 6 of the IGBT 7 is effectively utilized to ensure power savings of a semiconductor device on the whole. FIG. 17 is a view showing a configuration of a semiconductor device in the third background art. As shown in FIG. 17, the semiconductor device of the third background art comprises a MOSFET 17 which is an insulated gate transistor, a pulse signal source 24 for applying a pulse signal to a gate of the MOSFET 17, diodes 18 to 20, an inductor 21, a load 23 and a capacitor 22 for providing energy to the load 23.

In the third background art shown in FIG. 17, the pulse signal source 24 applies a pulse signal to the gate of the MOSFET 17 through the diode 20, thereby switching the MOSFET 17. When the MOSFET 17 is in an off state, the gate input capacitance 6 of the IGBT 7 and the inductor 21 resonate, to move energy which is accumulated in the gate input capacitance 6 when the MOSFET 17 is in an on state to the capacitor 22 through the diode 19. Then, the energy accumulated in the capacitor 22 is supplied to the load 23. Thus, in the third background art, since the energy accumulated in the gate input capacitance 6 is reused, loss of the energy in the semiconductor device on the whole can be reduced to consequently ensure power savings of the semiconductor device on the whole. Further, almost the same technique as the third background art shown in FIG. 17 is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-163862.

The third background art, however, is not a technique to reduce the required power capacity of a power supply to drive the MOSFET 17, like the control power supplies 15 a and 15 b, e.g., a power supply for the pulse signal source 24 and reduce the size of the power supply. Further, in order to quickly move the energy of the gate input capacitance 6 to the capacitor 22, it is necessary to resonate the gate input capacitance 6 and the inductor 21 and in order for that, it is necessary that the signals supplied from the pulse signal source 24 to the gate of the MOSFET 17 should have constant frequency and duty. For this reason, the semiconductor device shown in FIG. 17 can not operate the MOSFET 17 with an arbitrary switching frequency.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductor device and a method of driving a transistor, in which electric charges accumulated in gate input capacitance of an insulated gate transistor are effectively utilized to reduce required power capacity of a power supply to drive the transistor and ensure power savings of the semiconductor device on the whole.

According to a first aspect of the present invention, the semiconductor device includes an insulated gate transistor, a driving unit and a capacitor.

The driving unit produces charge and discharge of gate input capacitance of the insulated gate transistor, and the capacitor is linked to the charge and the discharge produced by the driving unit to be selectively connected to a gate of the insulated gate transistor.

The capacitor is linked to the discharge produced by the driving unit to be connected to the gate, whereby the gate input capacitance supplies the capacitor with electric charges accumulated therein and the capacitor accumulates therein the electric charges supplied from the gate input capacitance.

The capacitor is linked to the charge produced by the driving unit to be connected to the gate, whereby the capacitor supplies the gate input capacitance with the electric charges accumulated therein and the gate input capacitance accumulates therein the electric charges supplied from the capacitor.

In the semiconductor device according to the first aspect, since the capacitor is linked to the discharge of the gate input capacitance provided by the driving unit to be connected to the gate of the insulated gate transistor, when the capacitor is connected to the gate of the transistor in advance of the discharge of the gate input capacitance produced by the driving unit, for example, the gate input capacitance supplies the capacitor with the electric charges accumulated therein and the capacitor accumulates therein the electric charges supplied from the gate input capacitance in advance of the charge of the gate input capacitance produced by the driving unit. Further, since the capacitor is linked to the charge of the gate input capacitance produced by the driving unit to be connected to the gate of the insulated gate transistor, when the capacitor is connected to the gate of the transistor in advance of the charge of the gate input capacitance produced by the driving unit, for example, the capacitor supplies the gate input capacitance with the electric charges accumulated therein and the gate input capacitance accumulates therein the electric charges supplied from the capacitor. As a result, when being charged by the driving unit, the gate input capacitance of the insulated gate transistor eventually accumulates the electric charges which are accumulated therein at the time of the discharge of the gate input capacitance produced by the driving unit. Therefore, since the electric charges are accumulated in the gate input capacitance to some degree when the driving unit begins charging the gate input capacitance, it is possible to reduce the amount of electric charges to be supplied to the gate input capacitance by the driving unit until the charge of the gate input capacitance is completed. As a result, it is possible to reduce the required power capacity of the power supply to allow the driving unit to supply the gate input capacitance with the electric charges.

Furthermore, since part of the electric charges which are accumulated in the gate input capacitance of the insulated gate transistor at the time of the discharge thereof are used to charge the gate input capacitance, in other words, the electric charges accumulated in the gate input capacitance are effectively used, it is possible to ensure power savings of the semiconductor device.

According to a second aspect of the present invention, the method of driving the transistor includes the steps of (a) and (b). In the step (a) discharge of gate input capacitance in an insulated gate transistor is produced, and in the step (b) charge of the gate input capacitance is produced.

The step (a) includes the steps of (c) and (d). In the step (c) electric charges accumulated in the gate input capacitance are supplied to the capacitor and the electric charges supplied by the gate input capacitance are accumulated in the capacitor, and in the step (d) electric charges remaining in the gate input capacitance are extracted after the step (c).

The step (b) includes the steps of (e) and (f). In the step (e) electric charges accumulated in the capacitor are supplied to the gate input capacitance and the electric charges supplied by the capacitor are accumulated in the gate input capacitance after the step (d), and in the step (f) further electric charges are supplied to the gate input capacitance after the step (e).

In the method of driving a transistor according to the second aspect, when being charged by the driving unit, the gate input capacitance of the insulated gate transistor eventually accumulates the electric charges which are accumulated therein before the discharge of the gate input capacitance. Therefore, since the electric charges are accumulated in the gate input capacitance to some degree when the step (f) is executed, it is possible to reduce the amount of electric charges to be supplied to the gate input capacitance until the charge of the gate input capacitance is completed. As a result, it is possible to reduce the required power capacity of the power supply to supply the gate input capacitance with the electric charges in the step (f).

Further, since part of the electric charges which are accumulated in the gate input capacitance of the insulated gate transistor at the time of the discharge thereof are used to charge the gate input capacitance, it is possible to effectively use the electric charges accumulated in the gate input capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 10 are views showing a configuration of a semiconductor device in accordance with a preferred embodiment of the present invention;

FIG. 11 is a view showing the characteristic of a driving current of an IGBT 7 in the semiconductor device in accordance with the preferred embodiment of the present invention;

FIG. 12 is a view showing a configuration of a variation of the semiconductor device in accordance with the preferred embodiment of the present invention;

FIG. 13 is a view showing a configuration of another variation of the semiconductor device in accordance with the preferred embodiment of the present invention;

FIG. 14 is a view showing a configuration of a semiconductor device in the first background art;

FIG. 15 is a view showing the characteristic of the semiconductor device in the first background art;

FIG. 16 is a view showing a configuration of a semiconductor device in the second background art; and

FIG. 17 is a view showing a configuration of a semiconductor device in the third background art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a view showing a configuration of a semiconductor device in accordance with the preferred embodiment of the present invention. As shown in FIG. 1, the semiconductor device of this preferred embodiment comprises the IGBT 7 which is an insulated gate transistor, the protection diode 8 working against a reverse voltage applied between the emitter and the collector of the IGBT 7, a driving unit 100 for driving the IGBT 7, semiconductor switches 2 a to 2 c, a timing control circuit 10 and capacitors 1 a and 1 b to be selectively connected the gate of the IGBT 7. The gate input capacitance 6 is parasitic capacitance generated structurally between the gate and the emitter of the IGBT 7.

The driving unit 100 comprises the gate driving circuit 3, the control power supply 15 a which outputs a voltage of +15 V, the control power supply 15 b which outputs a voltage of −15 V, the transistors 4 a and 4 b and the resistor 5. The gate driving circuit 3 has one input terminal and two output terminals. The input terminal is connected to the timing control circuit 10. Further, one of the output terminals of the gate driving circuit 3 is connected to a gate of the transistor 4 a and the other is connected to a gate of the transistor 4 b. The gate driving circuit 3 outputs two signals of different logical levels on the basis of a signal from the timing control circuit 10, to turn on the transistor 4 a and off the transistor 4 b or turn off the transistor 4 a and on the transistor 4 b. Furthermore, the gate driving circuit 3 brings its output terminals into high impedance, to turn off both the transistors 4 a and 4 b.

The collector of the transistor 4 a is connected to the control power supply 15 a and the emitter of the transistor 4 b is connected to the control power supply 15 b. An emitter of the transistor 4 a and a collector of the transistor 4 b are connected to each other. The emitter of the transistor 4 a and the collector of the transistor 4 b are connected through the resistor 5 to the gate of the IGBT 7.

The driving unit 100 having the above configuration drives the IGBT 7 by charging and discharging the gate input capacitance 6 of the IGBT 7. Specifically, when the gate driving circuit 3 turns the transistor 4 a on and turns the transistor 4 b off, the electric charges are supplied by the control power supply 15 a to the gate input capacitance 6, thereby charging the gate input capacitance 6. When the gate driving circuit 3 turns the transistor 4 a off and turns the transistor 4 b on, the electric charges accumulated in the gate input capacitance 6 are extracted by the control power supply 15 b, thereby discharging the gate input capacitance 6.

The semiconductor switch 2 a is connected to the control power supply 15 a of the driving unit 100, the gate of the IGBT 7 and one end of the capacitor 1 a. The semiconductor switch 2 b is connected to the other end of the capacitor 1 a, the semiconductor switch 2 c and the ground. The semiconductor switch 2 c is connected to the gate of the IGBT 7, one end of the capacitor 1 b and the semiconductor switch 2 b. Further, the other end of the capacitor 1 b is connected to the ground. In this case, the capacitors 1 a and 1 b are controlled by the semiconductor switches 2 a to 2 c and linked to the charge and discharge of the gate input capacitance 6 produced by the driving unit 100 to be selectively connected to the gate of the IGBT 7.

The timing control circuit 10 is connected to the semiconductor switches 2 a to 2 c, and the semiconductor switches 2 a to 2 c are controlled by signals transmitted from the timing control circuit 10. Specifically, the semiconductor switch 2 a connects one end of the capacitor 1 a to the control power supply 15 a, to the gate of the IGBT 7 or to neither the control power supply 15 a nor the gate of the IGBT 7 on the basis of the signal from the timing control circuit 10. The semiconductor switch 2 b connects the other end of the capacitor 1 a to the ground or to the semiconductor switch 2 c on the basis of the signal from the timing control circuit 10. The semiconductor switch 2 c connects one end of the capacitor 1 b to the semiconductor switch 2 b, to the gate of the IGBT 7 or to neither the semiconductor switch 2 b nor the gate of the IGBT 7 on the basis of the signal from the timing control circuit 10.

An anode of the protection diode 8 is connected to the emitter of the IGBT 7 and a cathode of the protection diode 8 is connected to the collector of the IGBT 7, whereby the diode 8 serves as a protection diode against the reverse voltage applied between the collector and emitter of the IGBT 7.

Next discussion will be made on an operation of the semiconductor device of this preferred embodiment, having the above configuration, referring to FIGS. 1 to 10. First, referring to FIG. 1, on the basis of a signal from the timing control circuit 10, the semiconductor switch 2 a connects one end of the capacitor 1 a to the control power supply 15 a, the semiconductor switch 2 b connects the other end of the capacitor 1 a to the semiconductor switch 2 c and the semiconductor switch 2 c connects one end of the capacitor 1 b to the semiconductor switch 2 b. As a result, between the control power supply 15 a and the ground, the capacitors 1 a and 1 b are connected in series to each other, thereby being charged. Further, at this time, the gate driving circuit 3 turns both the transistors 4 a and 4 b off on the basis of a signal from the timing control circuit 10.

FIGS. 2 to 4 shows a process for charging the gate input capacitance 6 of the IGBT 7. Referring to FIG. 2, after the capacitors 1 a and 1 b are charged by the control power supply 15 a, in order to charge the gate input capacitance 6 of the IGBT 7, the semiconductor switch 2 a connects one end of the capacitor 1 a to the gate of the IGBT 7 on the basis of a signal from the timing control circuit 10. With this connection, the capacitors 1 a and 1 b supply the gate input capacitance 6 of the IGBT 7 with part of the electric charges accumulated therein. Then, the gate input capacitance 6 accumulates the supplied electric charges. At this time, when the supply of the electric charges from the capacitors 1 a and 1 b to the gate input capacitance 6 is completed, the electric charges accumulated in the gate input capacitance 6 almost depends on the ratio of the composite capacitance of the capacitors 1 a and 1 b to the gate input capacitance 6. With the electric charges supplied by the capacitors 1 a and 1 b to the gate input capacitance 6, in other words, with the electric charges accumulated in the gate input capacitance 6, the gate voltage of the IGBT 7 increases. In this preferred embodiment, it is assumed that the gate voltage is, e.g., less than the threshold voltage when accumulation of the electric charges in the gate input capacitance 6 is completed and the IGBT 7 is not turned on with the electric charges accumulated in the gate input capacitance 6.

Next, referring to FIG. 3, after a predetermined time from the connection of the capacitor 1 a to the gate of the IGBT 7, the semiconductor switch 2 a disconnects the gate of the IGBT 7 from the capacitor 1 a on the basis of a signal from the timing control circuit 10. At this time, this one end of the capacitor 1 a is connected to neither the control power supply 15 a nor the gate of the IGBT 7. Next, referring to FIG. 4, the driving unit 100 further supplies the gate input capacitance 6 with electric charges, thereby charging the gate input capacitance 6. Specifically, the gate driving circuit 3 turns only the transistor 4 a on, on the basis of a signal from the timing control circuit 10. When the transistor 4 a is turned on, a voltage of +15 V is supplied from the control power supply 15 a to the gate of the IGBT 7. At this time, electric charges are supplied from the control power supply 15 a to the gate input capacitance 6, thereby producing a gate current. Then, as the gate input capacitance 6 is being charged by the driving unit 100, the gate voltage of the IGBT 7 increases and when the gate voltage of the IGBT 7 becomes not less than the threshold voltage, the IGBT 7 is turned on. After that, when the charge of the gate input capacitance 6 is completed and the gate voltage of the IGBT 7 becomes about +15 V, the gate current almost stops flowing.

Next, referring to FIG. 5, the gate driving circuit 3 of the driving unit 100 turns the transistor 4 a off on the basis of a signal from the timing control circuit 10. As a result, no voltage is applied to the gate of the IGBT 7.

FIGS. 6 to 8 shows a process for discharging the gate input capacitance 6 of the IGBT 7. Referring to FIG. 6, after both the transistors 4 a and 4 b are turned off, in order to discharge the gate input capacitance 6, the semiconductor switch 2 a connects one end of the capacitor 1 a to the gate of the IGBT 7, the semiconductor switch 2 b connects the other end of the capacitor 1 a to the ground and the semiconductor switch 2 c connects one end of the capacitor 1 b to the gate of the IGBT 7, on the basis of signals from the timing control circuit 10. As a result, the capacitors 1 a and 1 b are connected in parallel to each other and both the respective one ends of the capacitors 1 a and 1 b are connected to the gate of the IGBT 7. Then, the gate input capacitance 6 supplies the capacitors 1 a and 1 b with part of the electric charges accumulated therein and the capacitors 1 a and 1 b accumulate the supplied electric charges therein. At this time, when the supply of the electric charges from the gate input capacitance 6 to the capacitors 1 a and 1 b is completed, the electric charges accumulated in the capacitors 1 a and 1 b almost depend on the ratio of the composite capacitance of the capacitors 1 a and 1 b to the gate input capacitance 6, like in the operation of the semiconductor device of this preferred embodiment discussed referring to FIG. 2. With the electric charges supplied by the gate input capacitance 6 to the capacitors 1 a and 1 b, the gate voltage of the IGBT 7 decreases. In this preferred embodiment, it is assumed that the gate voltage is, e.g., not less than the threshold voltage at this time and the IGBT 7 is not turned off by supplying the capacitors 1 a and 1 b with the electric charges accumulated in the gate input capacitance 6.

Next, referring to FIG. 7, after a predetermined time from the connection of the capacitors 1 a and 1 b to the gate of the IGBT 7, the semiconductor switch 2 a disconnects the gate of the IGBT 7 from one end of the capacitor 1 a and the semiconductor switch 2 c disconnects the gate of the IGBT 7 from one end of the capacitor 1 b, on the basis of signals from the timing control circuit 10. At this time, this one end of the capacitor 1 a is connected to neither the control power supply 15 a nor the gate of the IGBT 7, and one end of the capacitor 1 b is connected to neither the semiconductor switch 2 b nor the gate of the IGBT 7.

Next, referring to FIG. 8, the driving unit 100 extracts the electric charges remaining in the gate input capacitance 6, thereby discharging the gate input capacitance 6. Specifically, the gate driving circuit 3 turns only the transistor 4 b on, on the basis of a signal from the timing control circuit 10. When the transistor 4 b is turned on, a voltage of −15 V is supplied from the control power supply 15 b to the gate of the IGBT 7. At this time, since the electric charges are accumulated in the gate input capacitance 6 of the IGBT 7, the accumulated electric charges are extracted by the control power supply 15 b, thereby producing a gate current in a direction reverse to that in the case of turning on the IGBT 7. Then, as the gate input capacitance 6 is being discharged, the gate voltage of the IGBT 7 decreases and when the gate voltage of the IGBT 7 becomes less than the threshold voltage, the IGBT 7 is turned off. After that, when the discharge of the gate input capacitance 6 is completed and the gate voltage of the IGBT 7 becomes about −15 V, the gate current almost stops flowing.

Next, referring to FIG. 9, the gate driving circuit 3 of the driving unit 100 turns the transistor 4 b off on the basis of a signal from the timing control circuit 10. Then, referring to FIG. 10, when the semiconductor switch 2 a connects one end of the capacitor 1 a which accumulates the electric charges supplied from the gate input capacitance 6 to the gate of the IGBT 7 on the basis of a signal from the timing control circuit 10, the capacitors 1 a and 1 b supply the gate input capacitance 6 of the IGBT 7 with part of the electric charges accumulated therein. The gate input capacitance 6 accumulates the supplied electric charges therein. With the electric charges accumulated in the gate input capacitance 6, the gate voltage of the IGBT 7 increases, but in this preferred embodiment, it is assumed that the gate voltage is, e.g., less than the threshold voltage at this time and the IGBT 7 is not turned on with the electric charges accumulated in the gate input capacitance 6. After that, in the semiconductor device of this preferred embodiment, by repeating the operation as discussed above referring to FIGS. 3 to 10 is repeated, the IGBT 7 performs a switching operation. Further, a series of processes shown in FIGS. 10, 3 and 4 corresponds to the process for charging the gate input capacitance 6, like the processes shown in FIGS. 2 to 4.

In the semiconductor device of this preferred embodiment performing the above-discussed operation, when the driving unit 100 discharges the gate input capacitance 6 of the IGBT 7 (see FIG. 8), the capacitors 1 a and 1 b are connected to the gate of the IGBT 7 in advance of the discharge, whereby the gate input capacitance 6 supplies part of the electric charges accumulated therein to the capacitors 1 a and 1 b and the capacitors 1 a and 1 b accumulate therein the supplied electric charges (see FIG. 6). Then, when the driving unit 100 charges the gate input capacitance 6 (see FIG. 4), the capacitor 1 a is connected to the gate of the IGBT 7 in advance of the charge, whereby the capacitors 1 a and 1 b supply the gate input capacitance 6 with part of the electric charges which are supplied from the gate input capacitance 6 and accumulated therein and the gate input capacitance 6 accumulates the supplied electric charges therein (see FIG. 10). In other words, when the driving unit 100 charges the gate input capacitance 6, the gate input capacitance 6 eventually accumulates part of the electric charges which are accumulated therein at the time of the discharge of the gate input capacitance 6 produced by the driving unit 100. For this reason, since the electric charges are accumulated in the gate input capacitance 6 to some degree when the driving unit 100 begins charging the gate input capacitance 6, it is possible to reduce the amount of electric charges to be supplied to the gate input capacitance 6 by the driving unit 100 until the charge of the gate input capacitance 6 is completed. As a result, it is possible to reduce the required power capacity of the control power supply 15 a to allow the driving unit 100 to supply the gate input capacitance 6 with the electric charges.

Further, as to the control power supply 15 b, since part of the electric charges accumulated in the gate input capacitance 6 after the IGBT 7 is turned on are accumulated in the capacitors 1 a and 1 b when the driving unit 100 begins discharging the gate input capacitance 6, it is possible to reduce the amount of electric charges to be drained to the control power supply 15 b. As a result, it is possible to reduce the required power capacity of the control power supply 15 b.

Furthermore, in this preferred embodiment, unlike the above-discussed second background art, since part of the electric charges which are accumulated in the gate input capacitance 6 when the gate input capacitance 6 of the IGBT 7 is discharged are used to charge the gate input capacitance 6, in other words, the electric charges accumulated in the gate input capacitance 6 are effectively utilized, it is possible to ensure power savings in the semiconductor device.

Furthermore, in this preferred embodiment, unlike the above-discussed third background art, it is not necessary to operate the IGBT 7 always with a constant switching frequency in order to effectively use the electric charges accumulated in the gate input capacitance 6 and it is therefore possible to ensure power savings in the semiconductor device even when the IGBT 7 is operated with an arbitrary switching frequency.

FIG. 11 is a view showing a relation between a driving current I2 of the IGBT 7 in this preferred embodiment and a driving current I1 of the IGBT 7 in the first background art. The driving currents I1 and I2 shown in FIG. 11 are the gate currents supplied from the control power supply 15 a when the IGBT 7 is turned on. As shown in FIG. 11, in the semiconductor device of this preferred embodiment, the peak value of the driving current of the IGBT 7 and the total amount of the driving currents flowing until the IGBT 7 is turned on are reduced as compared with those of the first background art. This suggests that it is possible to reduce the required power capacity of the control power supply 15 a and ensure power savings in the semiconductor device.

Further, in this preferred embodiment, though the timing control circuit 10, the semiconductor switches 2 a to 2 c and the capacitors 1 a and 1 b are added to the configuration of the above first background art, these additional circuits can be achieved by using integrated circuits. The physical size of the control power supplies 15 a and 15 b mainly depends on a switching transformer and an electrolytic condenser. In this preferred embodiment, since the required power capacity of the control power supplies 15 a and 15 b can be reduced, it is possible to reduce the physical size of the switching transformer and the electrolytic condenser. A decrease in size of the switching transformer and the electrolytic condenser usually has a greater effect on the physical size of the semiconductor device on the whole than an increase in size by the additional circuits. For this reason, since the required power capacity of the control power supplies 15 a and 15 b can be reduced in the semiconductor device of this preferred embodiment, it is possible to reduce the size of the semiconductor device on the whole.

Furthermore, a decrease in physical size of the switching transformer and the electrolytic condenser usually has a greater effect on the cost of the semiconductor device on the whole than an increase in size by the additional circuits. In the semiconductor device of this preferred embodiment, since the required power capacity of the control power supplies 15 a and 15 b can be reduced in the semiconductor device of this preferred embodiment, it is possible to reduce the cost of the semiconductor device on the whole.

In this preferred embodiment, since the gate input capacitance 6 is charged by the capacitors 1 a and 1 b, if the capacitors 1 a and 1 b are regarded as a kind of power supply, the output impedance of this power supply is usually lower than the output impedance of the driving unit 100. Since the capacitors 1 a and 1 b supply the gate input capacitance 6 with the electric charges accumulated therein in advance of the charge of the gate input capacitance 6 produced by the driving unit 100, in other words, since the initial charge of the gate input capacitance 6 is produced by the capacitors 1 a and 1 b, the rise time of the gate voltage of the IGBT 7 can be reduced as compared with the first background art where all the charges of the gate input capacitance 6 are produced by the control power supply 15 a. Therefore, the switching operation of the IGBT 7 becomes faster than that in the first background art.

Further, though the link of the connection between the gate of the IGBT 7 and the capacitors 1 a and 1 b to the discharge of the gate input capacitance 6 produced by the driving unit 100 is established by connecting the capacitors 1 a and 1 b to the gate of the IGBT 7 in advance of the discharge of the gate input capacitance 6 produced by the driving unit 100 (see FIG. 6) in this preferred embodiment, the link of the connection between the gate of the IGBT 7 and the capacitors 1 a and 1 b to the discharge of the gate input capacitance 6 produced by the driving unit 100 may be established by connecting the capacitors 1 a and 1 b to the gate of the IGBT 7 on the way of discharging the gate input capacitance 6 by the driving unit 100. Furthermore, though the link of the connection between the gate of the IGBT 7 and the capacitor 1 a to the charge of the gate input capacitance 6 produced by the driving unit 100 is established by connecting the capacitor 1 a to the gate of the IGBT 7 in advance of the charge of the gate input capacitance 6 produced by the driving unit 100 (see FIGS. 2 and 10) in this preferred embodiment, the link of the connection between the gate of the IGBT 7 and the capacitor 1 a to the charge of the gate input capacitance 6 produced by the driving unit 100 may be established by connecting the capacitor 1 a to the gate of the IGBT 7 on the way of charging the gate input capacitance 6 by the driving unit 100.

Specifically, for example, the driving unit 100 discharges the gate input capacitance 6, and when the gate voltage decreases till near the threshold voltage, both the transistors 4 a and 4 b are turned off to stop the discharge. Then, when the capacitors 1 a and 1 b are connected to the gate of the IGBT 7, the gate input capacitance 6 supplies the capacitors 1 a and 1 b with part of the electric charges accumulated therein. When the supply of the electric charges to the capacitors 1 a and 1 b is completed, the capacitors 1 a and 1 b are disconnected from the gate of the IGBT 7 and the driving unit 100 produces the discharge of the gate input capacitance 6 again until it is completed.

Further, the driving unit 100 charges the gate input capacitance 6, and when the gate voltage increases till near the threshold voltage, both the transistors 4 a and 4 b are turned off to stop the charge. Then, when the capacitor 1 a which is connected in series to the capacitor 1 b is connected to the gate of the IGBT 7, the capacitors 1 a and 1 b supply the gate input capacitance 6 with part of the electric charges accumulated therein and the gate input capacitance 6 accumulates therein the supplied electric charges. When the accumulation of the electric charges into the gate input capacitance 6 is completed, the capacitor 1 a is disconnected from the gate of the IGBT 7 and the driving unit 100 produces the charge of the gate input capacitance 6 again until it is completed.

In the case where the electric charges accumulated in the gate input capacitance 6 are supplied to the capacitors 1 a and 1 b on the way of discharging the gate input capacitance 6 by the driving unit 100, however, since part of the electric charges of the gate input capacitance 6 are extracted by the driving unit 100, it is not possible to supply a sufficient amount of electric charges to the capacitors 1 a and 1 b as compared with the case of this preferred embodiment. In other words, in this preferred embodiment, since part of the electric charges accumulated in the gate input capacitance 6 are supplied to the capacitors 1 a and 1 b in advance of the discharge of the gate input capacitance 6 produced by the driving unit 100, it is possible to supply a sufficient amount of electric charges to the capacitors 1 a and 1 b. Therefore, the capacitors 1 a and 1 b can accumulate a sufficient amount of electric charges, and when the driving unit 100 begins charging the gate input capacitance 6, a sufficient amount of electric charges can be accumulated in the gate input capacitance 6. As a result, it is possible to surely reduce the amount of electric charges to be supplied to the gate input capacitance 6 by the driving unit 100 until the charge of the gate input capacitance 6 is completed and therefore the required power capacity of the control power supply 15 a can be surely reduced.

Further, in the case where the gate input capacitance 6 accumulates the electric charges supplied from the capacitors 1 a and 1 b on the way of charging the gate input capacitance 6 by the driving unit 100, however, since the gate input capacitance 6 are supplied with the electric charges by the driving unit 100, it is not possible to accumulate a sufficient amount of electric charges supplied from the capacitors 1 a and 1 b in the gate input capacitance 6 as compared with the case of this preferred embodiment. In other words, in this preferred embodiment, since the gate input capacitance 6 accumulates the electric charges supplied from the capacitors 1 a and 1 b in advance of the charge of the gate input capacitance 6 produced by the driving unit 100, it is possible to accumulate a sufficient amount of electric charges supplied from the capacitors 1 a and 1 b in the gate input capacitance 6. Therefore, when the driving unit 100 begins charging the gate input capacitance 6, sufficient amount of electric charges are accumulated in the gate input capacitance 6. As a result, it is possible to surely reduce the amount of electric charges to be supplied to the gate input capacitance 6 by the driving unit 100 until the charge of the gate input capacitance 6 is completed and therefore the required power capacity of the control power supply 15 a can be surely reduced.

In the above-discussed preferred embodiment, referring to FIG. 3, in the case of turning on the IGBT 7, though the gate of the IGBT 7 is disconnected from the capacitor 1 a after a predetermined time from the connection between the capacitor 1 a and the gate of the IGBT 7, it is desirable that this predetermined time should be not more than 5 μs. In other words, it is desirable that the time period from the connection of the gate of the IGBT 7 to the capacitor 1 a (see FIGS. 2 and 10) to the disconnection of the gate of the IGBT 7 from the capacitor 1 a (see FIG. 3) should be not more than 5 μs. Further, in the above-discussed preferred embodiment, referring to FIG. 7, in the case of turning off the IGBT 7, though the gate of the IGBT 7 is disconnected from the capacitors 1 a and 1 b after a predetermined time from the connection between the capacitors 1 a and 1 b and the gate of the IGBT 7, it is also desirable that this predetermined time should be not more than 5 μs. In other words, it is desirable that the time period from the connection of the gate of the IGBT 7 to the capacitors 1 a and 1 b (see FIG. 6) to the disconnection of the gate of the IGBT 7 from the capacitors 1 a and 1 b (see FIG. 7) should be not more than 5 μs.

The switching frequency fc of the IGBT 7 is usually about 2 to 20 kHz, and the requirements specification on the rise time or the fall time of the gate voltage of the IGBT 7 in a switching operation is not more than 5 μs. Accordingly, by setting the time period from the connection of the gate of the IGBT 7 to the capacitor 1 a to the disconnection of the gate of the IGBT 7 from the capacitor 1 a to be not more than 5 μs, it is possible to approximate the rise time of the gate voltage of the IGBT 7 in the switching operation to the requirements specification. Also by setting the time period from the connection of the gate of the IGBT 7 to the capacitors 1 a and 1 b to the disconnection of the gate of the IGBT 7 from the capacitors 1 a and 1 b to be not more than 5 μs, it is possible to approximate the fall time of the gate voltage of the IGBT 7 in the switching operation to the requirements specification.

In this preferred embodiment, referring to FIG. 2, the gate input capacitance 6 accumulates the electric charges supplied from the capacitors 1 a and 1 b in advance of being charged by the driving unit 100. Therefore, the gate voltage of the IGBT 7 increases but the gate voltage becomes less than the threshold voltage when the accumulation of the electric charges in the gate input capacitance 6 is completed. In other words, the IGBT 7 is not turned on with this gate voltage. In order to ensure power savings, however, it is desirable that the gate voltage of the IGBT 7 should become not less than the threshold voltage with the electric charges supplied by the capacitors 1 a and 1 b to the gate input capacitance 6.

Further, in this preferred embodiment, referring to FIG. 6, the gate input capacitance 6 supplies the capacitors 1 a and 1 b with part of the electric charges accumulated therein in advance of being discharged by the driving unit 100. Therefore, the gate voltage of the IGBT 7 decreases but the gate voltage becomes not less than the threshold voltage after the supply of the electric charges by the gate input capacitance 6 to the capacitors 1 a and 1 b. In other words, the IGBT 7 is not turned off with the gate voltage. In order to ensure power savings, however, it is desirable that the gate voltage should become less than the threshold voltage by supplying part of the electric charges accumulated in the gate input capacitance 6 to the capacitors 1 a and 1 b.

Usually, not less than about 95% of the total amount of charging currents of the IGBT 7 is the total amount of gate currents after the gate input capacitance 6 begins increasing by the Miller effect of the IGBT 7. Since the Miller effect is caused by turning on the IGBT 7, the gate input capacitance 6 accumulates the electric charges supplied from the capacitors 1 a and 1 b and the gate voltage of the IGBT 7 becomes not less than the threshold voltage, to compensate most of the charging currents with the electric charges accumulated in the capacitors 1 a and 1 b. Therefore, the gate voltage of the IGBT 7 becomes not less than the threshold voltage with the electric charges supplied from the capacitors 1 a and 1 b to the gate input capacitance 6, to thereby ensure further power savings.

Further, like the charging currents, most of the total amount of discharging currents of the IGBT 7 is usually the total amount of gate currents flowing while the Miller effect occurs in the IGBT 7, in other words, until the IGBT 7 is turned off. Therefore, part of the electric charges accumulated in the gate input capacitance 6 is supplied to the capacitors 1 a and 1 b and the gate voltage becomes less than the threshold voltage, to thereby accumulate most of the discharging currents in the capacitors 1 a and 1 b. Thus, since the electric charges accumulated in the gate input capacitance 6 can be effectively used, it is possible to ensure further power savings.

In order to achieve the above conditions, for example, the capacitances of the capacitors 1 a and 1 b should be each set to be twice as much as the gate input capacitance 6 at the time when the Miller effect occurs. In this preferred embodiment, with this setting, when the capacitors 1 a and 1 b are supplied with the electric charges accumulated in the gate input capacitance 6 and accumulates the supplied electric charges therein (see FIG. 6) after the charge of the gate input capacitance 6 of the IGBT 7 is completed by the driving unit 100 (see FIG. 4), the composite capacitance of the capacitors 1 a and 1 b which are connected in parallel to each other becomes four times as much as the gate input capacitance 6 at the time when the Miller effect occurs. Therefore, even if suppose that the Miller effect occurs even when the IGBT 7 is turned off, when the supply of the electric charges from the gate input capacitance 6 is completed, the gate voltage of the IGBT 7 becomes 3 V. Since the threshold voltage for the gate voltage is usually about 6 V, by supplying part of the electric charges accumulated in the gate input capacitance 6 to the capacitors 1 a and 1 b, the gate voltage becomes less than the threshold voltage.

Further, though the electric charges accumulated in the capacitors 1 a and 1 b are supplied to the gate input capacitance 6 (see FIG. 10) after the discharge of the gate input capacitance 6 of the IGBT 7 is completed by the driving unit 100 (see FIG. 8) in this preferred embodiment, the capacitors 1 a and 1 b may be always charged by the control power supply 15 a after the discharge of the gate input capacitance 6 of the IGBT 7 is completed by the driving unit 100. Specifically, as discussed with reference to FIG. 1, the capacitor 1 a is connected to the control power supply 15 a and the capacitors 1 a and 1 b are connected in series to each other between the control power supply 15 a and the ground. Thus, since the electric charges accumulated in the capacitors 1 a and 1 b are supplied to the gate input capacitance 6 after the capacitors 1 a and 1 b are charged and the capacitors 1 a and 1 b are connected in series to each other when the gate input capacitance 6 accumulates the supplied electric charges, the composite capacitance of the capacitors 1 a and 1 b are equal to the gate input capacitance 6 at the time when the Miller effect occurs. Therefore, when the accumulation of the electric charges in the gate input capacitance 6 is completed, the gate voltage of the IGBT 7 becomes 7.5 V. Since the threshold voltage for the gate voltage is usually about 6 V, as mentioned above, the gate voltage of the IGBT 7 becomes not less than the threshold voltage with the electric charges supplied from the capacitors 1 a and 1 b to the gate input capacitance 6.

Furthermore, in order only to make the gate voltage less than the threshold voltage, it is only necessary to set the composite capacitance of the capacitors 1 a and 1 b to be not less than twice as much as the gate input capacitance 6 at the time when the Miller effect occurs, and then the gate voltage becomes less than the threshold voltage by supplying part of the electric charges accumulated in the gate input capacitance 6 to the capacitors 1 a and 1 b. Specifically, even if suppose that the Miller effect occurs even when the IGBT 7 is turned off, when the supply of the electric charges from the gate input capacitance 6 is completed, the gate voltage of the IGBT 7 becomes not more than 5 V, in other words, less than the threshold voltage for the gate voltage.

In the switching operation of the IGBT 7, when the IGBT 7 is turned on by accumulating the electric charges supplied from the capacitors 1 a and 1 b in the gate input capacitance 6 and the IGBT 7 is turned off by supplying part of the electric charges accumulated in the gate input capacitance 6 to the capacitors 1 a and 1 b, the equivalent gate input capacitance 6 of the IGBT 7 increases since the gate of the IGBT 7 is connected to the capacitors 1 a and 1 b. Therefore, it is possible to reduce variation of the gate voltage caused by variation of a collector voltage which is generated when the IGBT 7 is turned on/off. As a result, it is possible to provide a semiconductor device performing a stable operation.

Though two capacitors 1 a and 1 b are used as capacitors for accumulating the electric charges accumulated in the gate input capacitance 6 in this preferred embodiment, a semiconductor device using only one capacitor as shown in FIG. 12 or a semiconductor device using three capacitors as shown in FIG. 13 can perform the same operation as that of this preferred embodiment by appropriately controlling the semiconductor switches 2 a to 2 e and the gate driving circuit 3 with the timing control circuit 10.

Further, though the capacitors 1 a and 1 b are connected in series to each other when the gate input capacitance 6 is charged by the driving unit 100 and the capacitors 1 a and 1 b are connected in parallel to each other when the gate input capacitance 6 is discharged by the driving unit 100 in this preferred embodiment, selection may be made on whether series connection or parallel connection according to the requirements specification of the semiconductor device.

Furthermore, though the capacitor 1 a is connected to the control power supply 15 a for supplying the gate input capacitance 6 with the electric charges and the capacitor 1 b is connected to the ground in this preferred embodiment, the capacitors 1 a and 1 b may be connected to a constant-voltage power supply different from the control power supply 15 a.

While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention. 

What is claimed is:
 1. A semiconductor device comprising: an insulated gate transistor; a driving unit for producing charge and discharge of gate input capacitance of said insulated gate transistor; and at least one capacitor linked to said charge and said discharge produced by said driving unit to be selectively connected to a gate of said insulated gate transistor, wherein said at least one capacitor is linked to said discharge produced by said driving unit to be connected to said gate, whereby said gate input capacitance supplies said at least one capacitor with electric charges accumulated therein and said at least one capacitor accumulates therein said electric charges supplied from said gate input capacitance, and said at least one capacitor is linked to said charge produced by said driving unit to be connected to said gate, whereby said at least one capacitor supplies said gate input capacitance with said electric charges accumulated therein and said gate input capacitance accumulates therein said electric charges supplied from said at least one capacitor.
 2. The semiconductor device according to claim 1, wherein said at least one capacitor is connected to said gate in advance of said discharge produced by said driving unit and said gate input capacitance supplies said electric charges accumulated therein to said at least one capacitor.
 3. The semiconductor device according to claim 1, wherein said at least one capacitor is connected to said gate in advance of said charge produced by said driving unit and said gate input capacitance accumulates therein said electric charges supplied by said at least one capacitor.
 4. The semiconductor device according to claim 2, wherein said at least one capacitor is connected to said gate in advance of said charge produced by said driving unit and said gate input capacitance accumulates therein said electric charges supplied by said at least one capacitor.
 5. The semiconductor device according to claim 1, wherein a gate voltage of said insulated gate transistor becomes not less than a threshold voltage with said electric charges supplied to said gate input capacitance by said at least one capacitor.
 6. The semiconductor device according to claim 1, wherein a gate voltage of said insulated gate transistor becomes less than a threshold voltage by supplying said at least one capacitor with said electric charges accumulated in said gate input capacitance.
 7. The semiconductor device according to claim 5, wherein a gate voltage of said insulated gate transistor becomes less than a threshold voltage by supplying said at least one capacitor with said electric charges accumulated in said gate input capacitance.
 8. The semiconductor device according to claim 1, wherein the capacitance of said at least one capacitor is not less than twice as much as said gate input capacitance at the time when the Miller effect occurs.
 9. The semiconductor device according to claim 1, wherein said at least one capacitor consists of a plurality of capacitors, and said plurality of capacitors are connected in parallel to one another when said electric charges accumulated in said gate input capacitance are supplied thereto and the plurality of capacitors are connected in series to one another when said electric charges accumulated therein are supplied to said gate input capacitance.
 10. A method of driving a transistor, comprising the steps of: (a) producing discharge of gate input capacitance in an insulated gate transistor; and (b) producing charge of said gate input capacitance, wherein said step (a) includes the steps of: (c) supplying electric charges accumulated in said gate input capacitance to said capacitor and accumulating said electric charges supplied by said gate input capacitance in said capacitor; and (d) extracting electric charges remaining in said gate input capacitance after said step (c), and said step (b) includes the steps of: (e) supplying electric charges accumulated in said capacitor to said gate input capacitance and accumulating said electric charges supplied by said capacitor in said gate input capacitance after said step (d); and (f) further supplying electric charges to said gate input capacitance after said step (e).
 11. The method of driving a transistor according to claim 10, further comprising the step of: (g) charging said capacitor after said step (a) and before said step (b).
 12. The method of driving a transistor according to claim 10, wherein said gate input capacitance supplies said electric charges accumulated therein to said capacitor after a gate of said insulated gate transistor and said capacitor are connected to each other in said step (c), said electric charges remaining in said gate input capacitance are extracted in said step (d) after said gate and said capacitor are disconnected from each other in said step (c), said capacitor supplies said electric charges accumulated therein to said gate input capacitance after said gate and said capacitor are connected to each other in said step (e), and said electric charges are further supplied to said gate input capacitance in said step (f) after said gate and said capacitor are disconnected from each other in said step (e).
 13. The method of driving a transistor according to claim 12, wherein a time period from connection of said gate to said capacitor in said step (c) to disconnection of said gate from said capacitor in said step (d) is not more than 5 .mu.s.
 14. The method of driving a transistor according to claim 12, wherein a time period from connection of said gate to said capacitor in said step (e) to disconnection of said gate from said capacitor in said step (f) is not more than 5 .mu.s.
 15. The method of driving a transistor according to claim 13, wherein a time period from connection of said gate to said capacitor in said step (e) to disconnection of said gate from said capacitor in said step (f) is not more than 5 .mu.s. 